This invention relates generally to MOSFET devices and more particularly to a simplified process for doping CMOS devices to more easily and efficiently regulate their threshold voltage characteristics.
Designing MOS devices that possess adequate threshold voltage behavior at both long and short channel lengths is a difficult process because many of the process components that affect threshold voltage interact at short channel lengths. A conventional process flow involves two steps. A first VT-shift implantation step is performed to fix the threshold voltage (V.sub.T) at long channel lengths. Secondly, a punchthrough implantation step is performed which prevents punchthrough and mitigates roll-off at short channel-lengths. "Roll off" is a term used to describe the reduction of threshold voltage with decreasing gate length due to the interaction of the source and drain regions of a MOSFET. An example of this effect is shown by line 86 in FIG. 7. Typically, this last step is accomplished using a high-angled implant of four discrete rotations performed after the gate of the CMOS device is etched.
This process is further complicated in CMOS technology where surface NMOS and PMOS transistors exist on the same wafer. Both devices are desired to have a particular threshold voltage where the characteristic behavior of the threshold voltage is predictably even over a range of channel lengths. Because the charge carrying mechanism is different in the two devices, it is impossible to induce a suitable threshold voltage for each device using a single mask and implant step. For instance, for a N-channel device, an implant of p-type dopant is required whereas for P-channel devices, an implant of n-type dopant is required.
Each of these VT-shift and masking steps increases the manufacturing costs for CMOS-based integrated circuits.
In addition to differences between N- and PMOS devices, there are also different problems associated with long and short channel devices.
Accordingly, the process steps used in conventional systems to stabilize the threshold voltage has traditionally been different. For instance, short channel devices suffer from "roll-off" where the threshold voltage drops sharply with decreasing channel lengths where channel-length can never be precisely controlled from device to device. Thus, in a 0.35 micron process, the MOSFET devices on any given wafer might have channel lengths which can vary from between 0.32 to 0.38 microns. If there is a precipitous roll-off in the threshold voltage, the threshold voltages can vary from between 0.3 to 0.5 volts. To reduce this so-called "short channel effect", punchthrough implants can be disposed adjacent the channel region of MOSFET devices according to a variety of well-known methods, thereby preventing a punchthrough of current at less than the desired threshold voltage for the device. At longer channel lengths, CMOS devices generally exhibit a drop off in their threshold voltage if punchthrough implants are used without corresponding VT-shift implants.
Accordingly, a need remains for a simplified universal process for setting the threshold voltage of various sizes and types of CMOS devices.